BES stops syncing

Hello!

My BES stops syncing since the last three weeks suddenly.

When I look in BAS under manage users, I can see that some users "last contact date" is up to date, but I also see that many other users "last contact date" is NOT up to date.
I solved this problem by restarting the BB controller service (but all services are started and running), after 5mins of waiting all devices "last contact date" are up to date and the users receive messsages on their devices again.

When I look at the windows event viewer I see a lot of BlackBerry Router Erros with Event ID 10000:

SERVICE_RELAY_SESSION:S34164325:0x0064EA28] Service transaction not found. SERVICESESSION_TAG=4506913